1. Field of the Invention
The present invention relates to a photographic optical system having a fixed focal length, and more particularly, to a photographic optical system for providing improved image quality at a variety of object distances through image processing.
2. Description of the Related Art
Generally, an optical system of a fixed focal length cannot properly perform a focusing function because a point spread function is seriously deteriorated as a camera approaches an object. Particularly, when a close-up shot is performed on an object distant away at about 10 cm, an image is very seriously deteriorated.
To solve this problem, an optical system for performing an auto-focusing function has been proposed. However, such an optical system requires transfer of a lens or an image sensor in order to perform the auto-focusing function, and thus a driving device for the lens or the image sensor is required. Therefore, an optical apparatus to which an optical system having an auto-focusing function is applied has an increased weight and a large volume.
Therefore, an apparatus or method has been proposed to obtain an image of excellent quality over a wide range of object distances through image processing without providing a driving device for an auto-focusing function.
U.S. Pat. No. 5,748,371 discloses a method and apparatus for increasing depth of field of an optical system using a mask.
As illustrated in FIG. 1, U.S. Pat. No. '371 proposes a method and apparatus for increasing depth of field by mounting a mask 20 on a conventional optical system including a lens 25 for forming an image of an object 15, an image sensor 30 for sensing the formed image, and an image processor 35.
At this point, the mask 20 is located between the object 15 and the lens 25 such that an optical transfer function (OTF) is not influenced by misfocus over a predetermined range of object distances.
The above optical system can have similar PSFs at variety of object distances using the mask 20, but a size of the PSF becomes relatively large and asymmetric (refer to P4 in FIG. 4), so that modulation transfer function at all object distances becomes considerably low (refer to FIG. 6b).
That is, according to the conventional optical system, an image quality is seriously reduced and a recovered image quality is poor.
Therefore, an optical system and an image processing method, capable of achieving an excellent image quality over a wide range of object distances without using a driving device in a conventional optical system of a fixed focal length, is highly required.